OK, I do not think that that can be done within B-3X (by transposing one of the keyboards).
If you are using the B-3X as a plug-in in a DAW you may be able to use a plug-in, placed before the B-3X, to transpose the MIDI on the lower zone (that is the lower notes) upwards. For a standalone B-3X, something like MIDI-OX (Windows only?) may transpose the notes.
http://www.midiox.com/https://www.sweetwater.com/sweetcare/articles/how-do-i-install-and-use-midi-ox-for-windows/Here is a plug-in, that I wrote for Reaper, but it can be used in other DAWs, on Windows, using the ReaPlugs package (
https://www.reaper.fm/reaplugs/ ).
The split is set to B2 and notes in the lower zone are transposed up by 3 octaves. The notes in the upper zone are not transposed. (You can also change the channel for notes in each Zone and use the graph to vary the velocities of the notes.)